When a fixed unit and a rotator communicate wirelessly with each other, circular polarization antennas mainly used for satellite communication and a radio frequency identification (RFID) are utilized as a transmitting antenna and a receiving antenna to prevent polarization mismatching in which polarized waves are orthogonal to each other, in spite of rotation, allowing appropriate communication at all rotation angles. However, the circular polarization antenna needs to generate two orthogonal polarized waves with the same amplitude and with 90-degree phase difference and thus has a complicated configuration.
On the other hand, linear polarization antennas and elliptic polarization antennas are mainly used for wide applications such as wireless local area networks (LANs) and can be more easily designed and manufactured than the circular polarization antenna. In view of design and manufacture costs, linear polarization antennas and elliptic polarization antennas are desirably also used for communication between the fixed unit and the rotator. As a technique using linear polarization antennas for communication between the fixed unit and the rotator, a technique is available in which a transmitting antenna is disposed in the rotator while a receiving antenna is disposed in the fixed unit so as to make a radiating element parallel to a rotation axis for wheels.
However, the antennas used for the above-described technique are assumed to be dipole antennas with rotationally symmetric radiation directivity. In both the transmitting antenna and the receiving antenna, a main radiating element is disposed along and parallel to the rotation axis. Thus, when the main radiating element is large in size, a large space is needed in a rotation axis direction to install the antenna. On the other hand, when the main radiating element is disposed in a plane perpendicular to the rotation axis direction, the space needed to install the antenna in the rotation axis direction can be made narrower. However, rotation may make polarized waves from the antennas in the fixed unit and the rotator orthogonal to each other. As a result, disadvantageously, polarization mismatching may occur, leading to a decrease in received power.